Paleoclimatic Study of the West Pacific Warm Pool using Oxygen Isotope Variations in a Stalagmite from Guam

The West Pacific Warm Pool (WPWP) is a region of the Pacific with mean seasonal sea surface temperatures > 28°C, and is a major driver of global climate. While the past ∼150 years have been instrumented and conditions have been recorded, it is unclear how much of the changes observed are due to anthropogenic effects or natural variability. In addition, this time frame may not include events that are natural but infrequent. This study investigates climate of the WPWP using oxygen isotope variations in speleothem calcite as a proxy for changes in rainfall on the island of Guam. The temperature effect on oxygen isotope ratios is relatively small in the tropics when compared to the effects of rainfall at Guam, and speleothem oxygen isotope ratios are more likely to vary in response to changes in storm tracks or rainfall amount. Previous research in the area has shown a correlation (r² of .57) between rainfall amount and stable isotopic composition. The speleothem sample "Stumpy" being used for this study was collected in 2005 from Jinapsan Cave on Guam (13° 38' N, 144° 53' E), where annual rainfall is connected to the state of the WPWP. Samples were milled at 1 millimeter resolution, representing an average resolution of 50 years per sample. U-series analyses show that the 33-cm-long Stumpy grew from ∼18,000 ka up to present day. Oxygen isotope values range from -9.4 to -4.5 per mille, and exhibit oscillations of approximately 0.5 per mille. At 27 cm, the record shows a 3.8 per mille shift to higher values, coincident with the occurrence of a dark brown growth layer. This shift may represent a change in rainfall across a growth hiatus. The range in oxygen isotope values of 4.9 per mille in Stumpy corresponds to a difference in rainfall of 350 +/- 74 mm/month, based on previous study of rainfall in the modern system.